1. Field of the Invention
The present invention relates to an electroluminescent element. More particularly, the present invention is concerned with an electroluminescent element comprising an anode, a cathode and, disposed therebetween, an organic luminescent layer comprising a mixture of a fluorescent luminescent agent, at least one hole moving and donating agent capable of moving holes and donating the same to the luminescent agent and at least one electron moving and donating agent capable of moving electrons and donating the same to the luminescent agent. In the element, the components of the luminescent layer have specific oxidation potential and reduction potential relationships therebetween. The element emits light in response to electrical signals.
The electroluminescent element of the present invention exhibits high luminescence efficiency and brightness, even at low voltages, and it can efficiently be produced at low cost.
2. Discussion of Related Art
Electroluminescent elements or devices are known, each of which comprises opposite electrodes and, disposed therebetween, an organic luminescent (light emitting) layer. Electrons are injected from one of the opposite electrodes, while holes are injected from the other of the opposite electrodes. When the injected electrons are recombined with the injected holes in the organic luminescent layer, light is emitted. In such an electroluminescent element, single crystalline anthracene and other single crystalline materials have been employed as an organic luminescent material for constructing the luminescent layer. The employment of single crystalline materials is however disadvantageous from the viewpoint of manufacturing cost and mechanical strength. Further, single crystalline materials inevictably have drawbacks in that a layer having an extremely small thickness is not easily formed, only a faint light is emitted with a single crystal having a thickness of about 1 mm and a driving voltage as high as 100 V or more is frequently required. Due to the above disadvantages and drawbacks, the single crystal materials have not yet been practically used in an electroluminescent element.
Attempts have been made to form a film of anthracene or the like having a thickness as small as 1 .mu.m or less by vapor deposition techniques (see Thin Solid Films, vol. 94, page 171, 1982). For a film to have desired performances, it is requisite that a thin film of only several thousand Angstroms or so in thickness be prepared under strictly controlled film-forming conditions. However, it should be noted that even if a luminescent layer is formed of such a thin film, the densities of holes and electrons as carriers are so low in the layer that the transportation and recombination of the carries cannot be satisfactorily accomplished, thereby causing efficient light emission to be unattainable. Especially, no satisfactory power consumption and brightness have been attained by only the use of such a thin film.
U.S. Pat. Nos. 4,356,429, 4,539,507 (corresponding to EP-A-120,673) and 4,720,432 (corresponding to EP-A-278,758) disclose electroluminescent elements in which a hole injecting layer is disposed between an anode and a luminescent layer in order to increase the density of holes as carriers and hence to obtain improved luminescence efficiency. In the electroluminescent elements of these patents, a material having excellent electron injecting and transporting properties as well as fluorescence efficiency must be used as the luminescent layer. However, no material disclosed therein is satisfactory in the above-mentioned properties and efficiency.
Moreover, Japanese Patent Application Laid-Open Specification Nos. 61-37886/1986, 2-250292/1990, 2-291696/1990 and 3-790/1991 disclose the use as a luminescent layer of a thin film of a mixture of a compound having fluorescence and having the capability of hole transportation and a compound having the capability of electron transportation, and also disclose the use, as a luminescent layer, of a thin film of a mixture of a compound having the capability of hole transportation and a compound having fluorescence and having the capability of electron transportation. That is, in the electroluminescent elements of these patent documents, a single compound serves to accomplish both the transportation of holes or electrons and light emission. However, any compound disclosed therein cannot satisfactorily perform both of the transportation of holes or electrons and the light emission at high efficiency. Accordingly, these conventional electroluminescent elements exhibit unsatisfactory brightness and power consumption.
Further, U.S. Pat. No. 4,769,292 discloses an electroluminescent element comprising in sequence an anode, an organic hole injecting and transporting layer, a luminescent layer and a cathode, in which the luminescent layer is formed of a thin film comprised of an organic host material forming a layer capable of sustaining both hole and electron injection and a small proportion of a fluorescent material. However, no material disclosed therein is satisfactory for performing injection of both the holes and electrons with high efficiency. Moreover, the transportation of holes and electrons to luminescence centers (fluorescent material), which is another step which is very important for obtaining high luminescence efficiency and brightness in the electroluminescent element, cannot satisfactorily be accomplished by any material described therein. Therefore, the disclosed electroluminescent element is unsatisfactory in respect of brightness and power consumption.